It has been shown that retinoids can block the phenotypic cell transformation produced by a polypeptide transforming factor, sarcoma growth factor (Todaro et al. Nature 276: 272-274, 1978). The purpose of this project is to isolate such transforming factors and then to investigate the mechanism whereby retinoids can interfere with their expression. The investigation will consist initially of two parts: first, isolation and purification of sarcoma growth factor from murine sarcoma virus-transformed cells in culture and from murine sarcoma virus-induced tumors in vivo and secondly, isolation and characterization of polypeptide transforming factors from carcinogen-induced tumors of epithelial tissues such as breast and bladder which have beeen shown to be responsive to retinoid therapy.